In a continuing program on the study of parasitic helmith-vitamin B12 relationships, the following aspects are being investigated: 1) a characterization of proteins as potential receptors for vitamin B12 on the tegumental surface of the plerocercoid (equals sparganum) of Spirometra mansonoides; 2) the effect of pH and various ions (Ca ions, Mg ions on the uptake of vitamine B12 by S. mansonoides; and 3) the ultrastructural characterization of the pseudocoelomocytes of Ascaris suum relative to vitamin B12 concentration mechanisms. The relationship of cholesterol to growth and differentiation has continued, investigating the effects of azasteroid, 25-azacoprostane, as a sterol inhibitor for the free-living stages of trichostrongylid nematodes (Nippostrongylus brasiliensis and Nematospiroides dubius). Radioactive cholesterol (14C) is also being used to study in vitro incorporation of the compound by larval stages. The homogonic free-living stages of Strongyloides venezuelensis was also found to have a requirement for growth and differentiation. Third stage larvae of the filariid, Dipetalonema viteae, are being studied in various media and gas phases for development in vitro.